1. Field of the Invention
The present invention relates to a hermetic type scroll compressor and a refrigerating and air-conditioning apparatus. Particularly, the present invention is suitably applicable to a hermetic type scroll compressor and a refrigerating and air-conditioning apparatus using carbon dioxide (CO2) as a refrigerant. Examples of a refrigerating and air-conditioning apparatus include equipment provided with a refrigeration cycle system such as air conditioners, refrigerators, and freezers.
2. Description of the Prior Art
As is well known, a scroll compressor element included in a hermetic type scroll compressor is mainly composed of a fixed scroll and a orbiting scroll each having a spiral scroll wrap erected on the corresponding end plate. In the scroll compressor element, the orbiting scroll, without rotating on its own axis relative to the fixed scroll, revolves orbitally at an approximately constant radius, thereby reducing the volume of a working chamber formed between the two scroll wraps and causing working fluid to be compressed. A hermetic type scroll compressor has such a scroll compressor element and an electrical motor element for driving the scroll compressor element both housed in a hermetic casing. Usually, the interior of the hermetic casing is kept at the suction pressure (low pressure) or the discharge pressure (high pressure) of the compressor.
In a scroll compressor using a low-pressure casing, that is, a hermetic casing whose interior is kept at the suction pressure of the compressor, lubricating oil contained in the suction gas changes in movement speed and volume inside the hermetic casing. This causes the lubricating oil to be separated in droplets from the suction gas and to flow into the working chamber in the scroll compressor element. Since the gas compressed in the working chamber flows out directly into an external refrigeration cycle system, sealing of the working chamber has to be maintained using a small amount of lubricating oil. It is therefore necessary to keep each of the gaps between the scroll wraps (axial gaps around end portions of the scroll wraps and radial gaps around sealed side portions of the scroll wraps) small, and doing so causes a problem of cost increase.
On the other hand, in a scroll compressor using a high-pressure casing, that is, a hermetic casing whose interior is kept at the discharge pressure of the compressor, lubricating oil contained in the discharge gas is discharged and separated in the hermetic casing, so that a relatively large amount of lubricating oil can be supplied to the working chamber. This makes it easy to control the gaps between the scroll wraps using oil film seals. Therefore, in the case of a scroll compressor using a high-pressure casing, the problem described above for a scroll compressor using a low-pressure casing does not occur, and it becomes possible to consider a cost reduction. In the case of a scroll compressor using a high-pressure casing, however, the hermetic casing is required to be made of thick material to secure adequate strength against pressure. Meeting the requirement causes a problem of a heavier compressor involving a cost increase.
In particular, from a viewpoint of global warming prevention, natural refrigerants with a small global warming potential have recently been attracting attention as refrigerants to take place of fluorocarbon refrigerants conventionally used for refrigeration systems. A CO2 refrigerant is regarded as having a promising future as a natural refrigerant for refrigerating and air-conditioning equipment. Compared with fluorocarbon refrigerants, the CO2 refrigerant has a low critical temperature of about 31° C., so that the operating pressure of the refrigeration system using the CO2 refrigerant becomes as high as about 10 MPa on the high-pressure side of the system. Therefore, a scroll compressor which uses a high-pressure casing and the CO2 refrigerant requires the casing to be made of particularly thick material. This leads to a problem of a heavier compressor involving a cost increase.
An example of a scroll compressor in which the interior of a hermetic casing is kept at an intermediate pressure between a suction pressure and a discharge pressure is disclosed in the specification of U.S. Pat. No. 4,343,599 (patent document 1). The patent document 1 describes a hermetic type scroll compressor in which the interior of a hermetic casing is kept at an intermediate pressure between a suction pressure and a discharge pressure and which includes an oiling passage and an oil return passage. The oiling passage is composed of a capillary tube. The opening at one end of the capillary tube is disposed in lubricating oil. The opening at the other end of the capillary tube communicates with a suction port of the compressor. The oil return passage is composed of a capillary tube. The opening at one end of the capillary tube communicates with an oil separator disposed on the discharge side of the compressor. The opening at the other end of the capillary tube communicates with the interior of the hermetic casing. According to the patent document 1, keeping the interior of the hermetic casing at an intermediate pressure between a suction pressure and a discharge pressure allows the strength against pressure of the casing to be low as compared with when a high-pressure casing is used, so that increases in weight and cost of the compressor can be suppressed.
Patent document 1: Specification of U.S. Pat. No. 4,343,599